Composite materials based on epoxy resins and inorganic or organic reinforcing agents have attained considerable importance in many areas of technology and daily life. This is due, on the one hand, to the fact that the epoxy resins can be processed relatively simply and safely and, on the other hand, to the good mechanical and chemical property level of the cured epoxy resin molded materials. This property level permits the epoxy resin materials to be adapted to diverse application purposes and allows the properties of the materials sharing in the composite to be used advantageously.
Epoxy resins are advantageously processed into composite materials via the manufacturing of prepregs. For this purpose, inorganic or organic reinforcing agents, respectively insertion components, in the form of fibers, fleece and fabric or textile materials are impregnated with the resin. In most cases, this is accomplished with a solution of the resin in an easily vaporizable or volatilizable solvent. The prepregs obtained after this process must not adhere anymore, but they also should not be cured yet; on the contrary the resin matrix should be merely in a prepolymerized state. In addition, the prepregs must be sufficiently stable in storage. Thus, for example, a storage stability of at least three months is required for manufacturing printed-circuit boards. In the subsequent processing into composite materials, the prepregs must furthermore fuse when there is a rise in temperature and must bind together under pressure with the reinforcing agents or insertion components as well as with the materials provided for the composite as compactly and permanently as possible; that is the cross-linked epoxy resin matrix must form a high degree of interfacial adherence with the reinforcing agents, as well as with the materials to be bonded together, such as metallic, ceramic, mineral and organic materials.
In the cured state, high mechanical and thermal stability as well as chemical resistance and heat deformation resistance or age resistance are generally required of the composite materials. For electrotechnical and electronic applications there is additionally the requirement for long-lasting, high electrical insulating properties. There are also a multitude of additional requirements for special application purposes. Required for the application as printed-circuit board material are, for example, a high-degree of dimensional stability over a broad temperature range, good adherence to glass and copper, high surface resistance, a low dielectric loss factor, good machining quality (punching capability, boring capability), minimal water absorption and high corrosion resistance.
With increasing stress and intensive use of the composite materials, the requirement for heat deformation resistance is given special importance. This means that during processing and use, the materials must withstand high temperatures without the composite becoming deformed or damaged, for example as the result of delamination. Printed-circuit boards, for example, are exposed to a temperature of 270.degree. C. during flow soldering. In the same way, during cutting and boring operations, temperatures of over 200.degree. C. can occur locally for brief periods. Materials with a high glass transition temperature T.sub.G exhibit favorable characteristics. If this glass transition temperature lies above the mentioned values, then inherent stability is guaranteed and damages such as warping and delamination are largely ruled out over the entire temperature range covered during processing. The epoxy resin currently used world-wide on a large scale for FR4 laminates has a glass transition temperature after curing of only 130.degree. C. This leads however to damages and breakdowns in manufacturing as described. Therefore, for quite some time there has been a need for cost-effective materials with comparatively good processibility and a glass transition temperature of over 200.degree. C.
A further requirement that has recently gained in importance is the requirement for flame resistance. In many areas, this requirement is given first priority--due to the danger to human beings and material assets--, for example in structural materials for airplane and motor vehicle construction and for public transportation vehicles. In electrotechnical and particularly electronic applications, it is absolutely necessary for the printed-circuit board materials to be flame resistant--due to the substantial worth of the electronic components assembled on them.
To assess burning behavior, one of the most demanding material testing standards must be withstood, namely the V-O rating according to UL 94V. In this test, a defined flame is applied vertically to the lower edge of a test piece. The sum of the combustion periods for ten tests must not exceed 50 s. This requirement is difficult to fulfill, especially when the wall thicknesses are thin, as is usually the case in electronics. The epoxy resin used in technical applications world-wide for FR4 laminates fulfills these requirements only because, as far as the resin is concerned, it contains approximately 30 to 40% core-brominated, aromatic epoxy components, that is approximately 17 to 21% bromine. For other application purposes, comparatively high concentrations of halogen compounds are used and are even often combined with antimony trioxide as a synergist. The difficulty associated with these compounds is that, on the one hand, they may be very effective as flameproofing agents, but, on the other hand, they possess very dangerous properties. Thus, antimony trioxide is on the list of carcinogenic chemicals. Also, in thermal decomposition, aromatic bromine compounds not only split off bromide radicals and hydrogen bromide, which lead to pronounced corrosion, but also when decomposing in the presence of oxygen, the highly brominated aromatic hydrocarbons in particular can also form highly toxic polybromodibenzofurans and polybromodibenzodioxins. Furthermore, there are considerable difficulties involved in the disposal of bromine-containing waste materials and toxic waste.
Materials, which comply with the requirement for increased resistance to heat deformation or even fulfill this requirement are for example molded materials based on bismaleinimid/triazine (BT) with a T.sub.G of approximately 200.degree. C. or polyimide (PI) with a T.sub.G of 260.degree. to 270.degree. C. Recently, BT epoxy blends with a T.sub.G of 180.degree. C. are also being offered. Laminates manufactured with these resin systems, however, exhibit worse processing and machining qualities than laminates based on epoxy resin. Thus, for example, manufacturing laminates on a polyimide base necessitates molding temperatures of about 230.degree. C. and considerably longer after-curing times (about eight hours) at temperatures of 230.degree. C. Other serious disadvantages of the polyimides are the low [level of] interlaminar adhesion, which causes delamination in die cutting operations and requires considerably more expensive and time-consuming machining processes, such as sawing or milling, as well as the high level of moisture absorption, which necessitates a costly drying operation (two hours at 120.degree. to 140.degree. C.) for the laminates and prepregs before the molding operation. In addition to this is the fact that in the machining of molded materials based on PI and BT, the borer is subject to greater wear resulting from the preparation of the fitting holes. Another serious disadvantage of these resin systems is that their material cost is six to ten times higher.
A comparatively cost-effective resin system is obtained when aromatic and/or heterocyclic polyepoxide resins, that is when polyglycidyl compounds are combined with aromatic polyamines as curing agents. These types of polyamines, known for example from the German Patent 27 43 680, result in network polymers that are particularly stable to heat-deformation and age resistant. From the European Published Patent Application 0 274 646, one can see that when 1.3.5-tris(3-amino-4alkylphenyl- 2.4.6-trioxo-hexahydrotriazines are used as curing agents, laminates are obtained, which exhibit glass transition temperatures of up to 245.degree. C. and are distinguished by a good processing and machining quality.
Even when the mentioned resin systems exhibit widely differing burning behaviors, the disadvantage of being inherently not flame retardant enough applies to all of them. Therefore, to fulfill the requirement indispensable for many application purposes, namely withstanding the fire test according to UL 94V with the rating V-O, one cannot do without the application of highly effective, bromine-containing flameproofing agents. The result is that one must put up with the potential danger associated with bromine compounds, on the one hand, and with a deterioration of the thermomechanical property level caused by the bromine compounds, on the other hand. It is known namely that adding core-brominated aromatic hydrocarbons brings about a reduction of the glass transition temperature. It was found, for example, that the glass transition temperature of polyimides is reduced by 40.degree. to 70.degree. C. when bromine compounds are added (c.f.:"Polymer Journal", vol. 20 (1988), pp 125 fol). In the epoxy and polyamine systems described in the European Published Patent Application 0 274 646, a reduction of the T.sub.G by approximately 50.degree. C. to values of below 200.degree. C. is found when the epoxy component is partially replaced by the corresponding brominated compound, meaning a total bromine concentration of 4%.
For these reasons, there have been many attempts to replace the bromine-containing flameproofing agent with less problematical substances. For example, fillers with a quenching gas effect were suggested, such as hydrated aluminum oxides (c.f.: "J. Fire and Flammability", vol. 3 (1972) , pp 51 fol) , alkaline aluminum carbonates (c.f.: "Plast. Engng.", vol. 32 (1976) , pp 41 fol) and magnesium hydroxides (European Published Patent Application 0 243 201), as well as vitrifying fillers, such as borates (c.f.: "Modern Plastics", vol. 47 (1970), No. 6 pp 140 fol) and phosphates (U.S. Pat. No. 2,766,139 and 3,398,019). Associated with all of these fillers is the disadvantage however, that in part they cause the mechanical, chemical and electrical properties of the composite materials to deteriorate considerably. Moreover, they require special, for the most part more expensive processing techniques, since they tend to sediment and because they increase the viscosity of the filled resin system.
The flame-retarding effectiveness of red phosphorus has also been previously described (British Patent 1,112,139), possibly combined with finely dispersed silicon dioxide or hydrated aluminum oxide (U.S. Pat. No. 3,373,135). Materials are thereby obtained which--due to the phosphoric acid that forms in the presence of moisture and the associated corrosion--limit the use for electrotechnical and electronic purposes. Furthermore, organic phosphorous compounds, such as phosphoric acid esters, phosphonic acid esters and phosphines, have been suggested as flame-retarding additives (c.f.: W.C. Kuryla and A.J. Papa "Flame Retardancy of Polymeric Materials", vol. 1, pp 24 to 38 and 52 to 61, Marcel Dekker Inc., New York, 1973). Since these compounds are known for their "plasticizing" properties and are used all over the world on a large scale as plasticizers (British Patent 10,794), this alternative is not very promising either.
It is also possible to use phosphorous compounds, such as epoxy-group-containing phosphorous compounds which are anchorable in the epoxy resin network, to regulate the flame-retardancy of epoxy resins. However, a great many of these compounds are liquid or crystalline, so that using them to regulate the flame-retardancy of epoxy resins is out of the question. This is because they either distinctly lower the softening point of the solid resins, cause stickiness, or--in the case of crystalline compounds--recrystallize (c.f.: German Published Patent Application 25 38 675). Therefore, in all known publications, epoxy-group-containing phosphorus compounds are first converted in a prereaction with bisphenols or phenolic resins (German Published Patent Application 25 38 675 and Japanese Published Patent Application 58-185631), with dicarboxylic acids (c.f.: "Vysokomol. Soedin.", Ser. B. 29 (1), pp 45 to 47 (1987)), or with polyester resins from dihydroxy compounds and dicarboxylic acids (Japanese Published Patent Application 50-043221). Only then are they cured--primarily ionically--in a mixture with the epoxy resins. However, the flame resistance that is attained is negligible. Thus, for example, for resins obtained through a prereaction with bisphenols, LOI values (Limiting Oxygen Index) of 23.5 to 24 are attained (c.f. German Published Patent Application 25 38 675). These are values that one would find combustible materials, such as wool (LOI=25), while the LOI values for known flame retardant materials, such as polysulphone (LOI=30), polyvinyl chloride (LOI=42), polyvinylidene chloride (LOI=60) and polytetrafluoroethylene (LOI=95), lie considerably higher (c.f.: D.W. v. Krevelen "Properties of Polymers", Elsevier Scientific Publishing Comp., Amsterdam, Oxford, New York, 1976, pp 525 fol). In the case of the conversion products of polyester resins with triglycidyl phosphates (c.f.: Japanese Published Patent Application 50-043221), even halogen compounds and antimony trioxide must be added to regulate the flame retardancy of polyester fibers.
The thermomechanical properties of the molded materials obtained during hardening, however, are likewise insufficient. Thus, for example, a T.sub.G of 45.degree. to 80.degree. C. is obtained during the aminic hardening of epoxy resins resulting from methyl diglycidyl phosphonate and dicarboxylic acids (c.f. "Vysokomol. Soedin.", Set. B. 29 (1), pp 45 to 47 (1987)). This conforms with the experience that phosphoric acid ester groupings, both in the main polymer chain as well as in the side chain, basically cause a plasticization and thus a lowering of the glass transition temperature (c.f. "Journal of Macromolecular Science", Vol. C1, No. 1 (1967), pp 1 fol.).
Therefore, it is an object of the invention to provide epoxy resin compounds for manufacturing prepregs and composite materials which can be obtained inexpensively and are comparable in processibility to epoxy resins found in technical applications.
It is a further object of the invention to provide epoxy resin compounds which yield inflammable molded materials or composite materials having a highest possible glass transition temperature (of &gt;&gt;200.degree. C.), and which can be rated at V-O according to UL 94V without having to add halogen compounds or antimony trioxide.